What is the mil rating of the vapor barrier that must be installed under a concrete slab?

• A. 1
• B. 4
• C. 6
• D. 2

Answer: (C)

Moisture from the soil can move up into a concrete slab, so a vapor barrier is placed under the slab to block that vapor. The mil rating measures the thickness of that plastic film—the higher the number, the more resistant and durable the barrier is. A 6-mil polyethylene sheet is the typical minimum used for sub-slab vapor barriers, offering a reliable balance of protection and cost. Thinner films (1, 2, or 4 mil) are more prone to punctures and tears, which can allow moisture to pass through. While thicker barriers exist for harsher conditions, 6 mil is the standard choice in most residential and landscape slab installations.